How do goods trains reach Ruislip depot?

Yes. That is the arrangement.

From the National Rail network trains are routed from the Down Main at West Ruislip into the Down Siding. The Marylebone South Signaller (as I understand it) will contact LUL and 'come to a clear understanding' as we say, but there is no slot on ME592 ground position-light signal which will be cleared for the route to the LUL Depot. You can see the layout on SimSig's plan:

This is a bit more up-to-date, but less official-looking than this photograph of Marylebone IECC taken in December 2007:

The signalling centre displays don't look like that anymore and the layout has been partially altered, with a new facing crossover between the Down and Up Main at the South Ruislip-end and some new signals.

The NR Sectional Appendix states that, before a movement is made from the Down Siding to the Connecting Line, the person in charge of the movement must obtain the permission of the LUL Signaller by means of a telephone provided for the purpose. Once permission has been granted (assuming the signal is off, of course) the movement may be propelled (assuming it isn't top and tailed) as far as the notice board lettered ‘Stop and Telephone LT Signaller’. Locomotives may pass the stop board provided the driver is conducted by a competent member of LUL staff. It then goes on to say:

This seems to assume the traditional method of working by which a locomotive would propel the vehicles onto somebody else's territory. The vehicles would then be secured, the locomotive would be detached, the vehicles would be left for the other company's locomotive to come and collect them and the original locomotive would return. Judging by the fact that the S Stocks were (I'm pretty sure) delivered in a top and tail formation I'm not sure that it always works like this in reality.

The instructions concerning exchange of traffic from LUL to Network Rail say only:

Again appearing to assume an NR locomotive will proceed into LUL territory, collect vehicles and draw them onto the Down Siding with the locomotive at the country end. Marylebone South would pull off ME589 GPL for the train to draw into the Down Sidings. From here, if the locomotive(s) are only at one end of the formation it will be necessary to propel from ME592 onto the Down Main as far as ME80 signal (the limit of shunt shown in the 2007 picture has been replaced with a main aspect signal). From here, if you were top and tailed, you could work wrong road all the way to South Ruislip and then towards Greenford (see SimSig's plan, this would not have been possible before). If not you can obviously proceed on your way quite happily in the down direction. Otherwise it would be possible to run-round at West Ruislip, but it would be a bit of a faff.